The invention relates to fiber Raman lasers, and more particularly to the production of short pulses in fiber Raman laser oscillators.
Fiber Raman lasers are described in the Handbook of Laser Science and Technology, Section 2: "Solid State Lasers", Subsection 2.4 "Fiber Raman Lasers", by R. H. Stolen and C. Lin, pages 265-273. A mode-locked YAG or Argon laser is used to synchronously pump a fiber, and the Raman effect in the fiber provides gain, in a fiber Raman oscillator. The output is shifted to the red; the pulse width is typically about the same as the pump pulse length for longer pump pulses while pulse broadening dominates for short pump pulses. By comparison, synchronously pumped dye lasers absorb light at one wavelength and emit light at another wavelength. When the dye absorbs a pump pulse, it emits a much shorter pulse. Thus with a 100 ps pump pulse it is possible to obtain 1 ps output pulses from a dye laser but it is not generally possible to obtain a shorter pulse from a fiber Raman oscillator. Shortening the pump pulse length to 1 ps will not produce subpicosecond pulse outputs in a fiber Raman oscillator since the fiber is dispersive and broadens the pulse; a typical 1OO m fiber produces many picoseconds of spreading or pulse broadening for a 1 ps pump pulse.
In order to prevent pulse spreading in a fiber Raman oscillator it is possible to operate near a single zero dispersion wavelength (about 1.57 microns) as demonstrated by M. N. Islam and L. F. Mollenauer at Bell Laboratories, IQEC 1986 Advance Program, page 45 and Lasers and Applications, May 1986, pages 66-67. However, this method imposes severe constraints of operating at a particular wavelength for which pump sources are not readily available and producing an output which may not match the desired application.
Another method of producing short pulses utilizes the pulse compression techniques of U.S. patent application Ser. No. 729,376 filed May 1, 1985. However, this method requires long fiber lengths and high pump power. Furthermore the output pulse characteristics are highly dependent on pump source intensity and pulse width.